With electrical contacts, whether in a high current circuit, or in the form of conventional relay output contacts or in other similar circuits, a common problem is the possible creation of an electrical arc between the contacts as they begin to open from a closed position. If the voltage across the opening contacts is allowed to rise to a sufficient level, an arc forms between the contacts. The voltage may even be sufficient that the arc will continue even after the contacts open and in an extreme case, the arc may continue even to maximum contact separation. Arcing is undesirable because of the wear it produces on the contacts as well as other circuit effects which may occur due to the arc current after the circuit should be open.
Typically, the manufacturers of devices such as relay contacts rate those contacts to switch a certain voltage and current reliably many thousands if not millions of times. To guarantee such a performance rating, the manufacturer typically relies on the inherent arc suppression and/or arc extinction characteristics of that particular contact arrangement. Characteristics which influence a contact's ability to suppress or extinguish an arc include the smoothness, size and shape of the contacts, the separation rate, the final maximum separation distance, and the characteristics of the medium separating the contacts in their open state.
These inherent arc suppression and/or extinction characteristics can be augmented by placing external components/circuitry across the contacts which hold the peak voltage or rate of increase of the voltage across the contacts to a value compatible with the separation rate or final maximum separation distance of the contacts. An example of such an external component is a capacitor. This technique is shown in U.S. Pat. No. 4,438,472 to Woodworth. Woodworth increases the effect of the shunting capacitor with a bipolar junction transistor.
Such a technique is not appropriate in many applications, however, including protective relays in a power substation. The capacitance may appear as a short circuit, even when the contacts are open. Further, for loads which are significantly less than the circuit is designed for, the time required for interrupting the load current is significantly extended.
Another approach involves the control of the peak voltage across the contacts without regard to their separation rate. The voltage is limited to a value in accordance with the rating of the contacts and the expected load current. This technique allows an arc to form but limits the peak voltage across the contacts such that the arc is extinguished by the natural characteristics of the particular contact arrangement. This technique, however, limits the operation of the contacts to rated performance which in many cases is impractical or otherwise unacceptable.